The northern leopard frogs that inhabit the boreal U.S. have never recovered from some catastrophic population declines in the 1970s. Some blame it on the acidifying lakes and streams caused by coal-burning, others point to the ongoing loss of wetlands to development, and now new evidence shows that the herbicide atrazine—widely sprayed on crop fields throughout the region—is killing the frogs by helping parasitic worms that feast on them.

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"Atrazine provides a double whammy to frogs: It increases both amphibian exposure and susceptibility," says biologist Jason Rohr of the University of South Florida in Tampa, who tested the impact by re-creating field conditions in 300-gallon (1,135-liter) tanks in his lab. "Atrazine is one of the more mobile and persistent pesticides being widely applied. In fact, residues have been found in remote, nonagricultural areas, such as the poles."

That may explain why amphibians are on the decline worldwide. As many as one third of the nearly 6,000 known amphibian species—frogs, toads, salamanders, even wormlike caecilians—are threatened with extinction, according to the International Union for Conservation of Nature (IUCN). And no one knows why.